1. Field of the Invention
The present invention relates to a brushless motor, such as a permanent-magnet small motor, a spindle motor and various micro motors, generally used for office automation equipment and the like.
2. Description of the Related Art
While brushless motors come in various configurations, most of the brushless motors are structured to include: a stator assembly which is composed such that a multi-phase winding is wound around each of pole teeth of a stator core having a plurality of slots; and a rotor assembly which includes an annular rotor magnet disposed to oppose the stator assembly and having opposite magnetic polarities alternately arranged in the circumferential direction and which is rotatably supported, wherein the multi-phase winding is energized in a selectively switching manner, whereby the rotor assembly is caused to rotate by the electromagnetic interaction between the current of the winding and the magnetic field of the rotor magnet.
In the brushless motors, the polarity and rotational position of the rotor magnet are detected by a sensor such as a Hall element or a voltage induced at the winding, whereby the timing of energization is switched in a controlled manner. The brushless motors fall into two types: an outer rotor type structured such that the rotor assembly is disposed outside the stator assembly; and an inner rotor type structured such that the rotor assembly is disposed inside the stator assembly.
A conventional outer rotor type brushless motor is disclosed, for example, in FIG. 1 of Japanese Patent No. 3524138. In the brushless motor, the number of slots of a stator core is a multiple of two (even number), more specifically an even number larger than four, and the number of magnetic poles of a rotor magnet is equal to the number of slots (that is to say, the number of pole teeth). Further, the brushless motor has a sensor disposed to axially face the rotor magnet and positioned radially outward of the slot between two arbitrary adjacent pole teeth of the stator core.
Windings individually wound around the pole teeth of the stator core form respective circuit networks, which are driven with substantially no turn-off time by a single-phase power supply and simultaneously by means of bidirectional energization where the direction of energization is reversed, wherein the windings are excited such that the electromagnetic pole of each winding formed by energization has a polarity opposite to that of the magnetic pole of the rotor magnet, which is positioned to oppose the electromagnetic pole of the relevant winding.
In the brushless motor described above, the number of the magnetic poles of the rotor magnet is equal to the number of the slots (pole teeth) of the stator core, and the windings disposed around the pole teeth of the stator core, when having a large number of winding turns, block reduction of the axial dimension, that is the overall height, of the motor.
FIG. 5 shows a conventional inner rotor type brushless motor 13 including: a stator assembly 7 which includes casings 11 and 12, a stator core 5 disposed inside the casings 11 and 12 and including a plurality (six in the example; refer to FIG. 6A) of pole teeth 1 each including, as its constituent segment, a tooth head 2 connected to the distal (inner) end thereof, and a plurality of windings 3 each wound around the pole tooth 1 of the stator core 5 so as to spread in the radial direction; and a rotor assembly 6 which includes a magnet 4, a magnet holding member 10 and a shaft 8, and which is rotatably supported by bearings 9 such that the magnet 4 radially opposes the tooth heads 2 of the pole teeth 1 of the stator core 5 with an air gap t therebetween.
In the single-phase brushless motor described above, the number of the pole teeth 1 of the stator core 5 in the stator assembly 7 is equal to the number of magnetic poles of the magnet 4 of the rotor assembly 6. Description will now be made of the stator core 5 included in the above conventional brushless motor 13 with reference to FIGS. 6A and 6B which respectively shows a configuration of the stator core 5 and an enlarged view of a portion of FIG. 6A indicated by A, and also with reference to FIGS. 7A and 7B which additionally show windings and a rotor assembly.
Referring to FIG. 6A, the stator core 5 includes six of the aforementioned pole teeth 1 (1a to 1f), and referring to FIGS. 7A and 7B, six of the aforementioned windings 3 (3a to 3f) are wound around the pole teeth 1a to 1f, respectively, and the magnet 4 of the rotor assembly 6 is disposed to oppose six of the aforementioned tooth heads 2 (2a to 2f) of the pole teeth 1a to 1f of the stator core 5.
The windings 3a to 3f wound around the pole teeth 1a to 1f are arranged in the circumferential direction such that a clockwise winding and a counterclockwise winding alternate with each other. The windings 3a to 3f are single-phase windings. The magnet 4 of the rotor assembly 6 has, at the radially outermost portion, six magnetic poles magnetized in the circumferential direction with N and S poles alternating with each other, is disposed to oppose the tooth heads 2a to 2f of the pole teeth 1a to 1f of the stator core 5 in the stator assembly 7, and is provided with the shaft 8, wherein the rotor assembly 6 is rotatably supported. In connection with FIGS. 7A and 7B, the symbol “x” in each of the windings 3a to 3f indicates that the wire comes toward the viewer, and the symbol “•” indicates that the wire goes away from the viewer.
Further, referring to FIG. 8 showing a cross section of the winding 3 wound around the pole tooth 1 of the stator core 5, a height (overall axial direction dimension) H of the winding 3 is represented by a formula: H=Y+2×W, where Y is a height (axial direction dimension) of the tooth pole 1, and W is a winding layer thickness of the winding 3.
According to the above formula, if the height Y of the pole tooth 1 is set to a fixed value, the height H of the winding 3 wound around the pole tooth 1 is determined by the winding layer thickness W of the winding 3, and therefore the winding 3 may possibly restrict reduction of the axial dimension of the brushless motor 13. When the winding 3 is wound with a large number of turns at a limited space, the height (axial direction dimension) H of the winding 3 is increased substantially thus hampering reduction of the axial dimension of the brushless motor 13, which is likewise seen with respect to the earlier described motor disclosed in Japanese Patent No. 3524138.
In the above conventional brushless motors disclosed in Japanese Patent No. 3524138 as well as described with reference to FIGS. 5 to 8, the reduction of the axial dimension (overall height) of the motor is blocked by the very presence of the winding wound around the pole tooth of the stator core, thus making it difficult to downsize the motor.